Prior art of similar technology will be described with reference to FIG. 3 and FIG. 4.
FIG. 3 is a half-sectional view showing a configuration of a conventional loudspeaker, and FIG. 4 is an enlarged sectional view of a configuration of a yoke that is an essential component.
In FIG. 3, magnetic circuit A comprises yoke 1 comprising cylindrical peripheral portion 1a and bottom yoke 1b, magnet 2, and top plate 3. Also, the bottom yoke 1b is provided with recess 1c at a top surface as shown in an enlarged view in FIG. 4. The recess 1c is formed to prevent an adhesive used for bonding the magnet 2 and the bottom yoke 1b from getting into magnetic gap 1d. 
An audio signal is input into voice coil 4 inserted into the magnetic gap 1d, to drive diaphragm 6 via the voice coil 4. The diaphragm 6 is held by a peripheral portion fixed on a frame and damper 7.
Generally, the cylindrical peripheral portion 1a of the yoke and the bottom yoke 1b of the yoke are formed of plate members having the same thickness and are integrally bonded or welded.
Recently, there has been an increasing trend of size-reduction and weight reduction even in in the field of various acoustic apparatus, and loudspeakers are also required to be smaller in size and higher in efficiency. Accordingly, a neodymium magnet having higher magnetic energy as compared with a ferrite magnet is already employed as the magnet 2, but with miniaturization of the magnetic circuit A, loudspeakers are required to be further enhanced in efficiency, and this becomes an important problem to be solved. For enhancing the loudspeaker efficiency, it is indispensable to improve the efficiency of the magnetic circuit.
In a conventional magnetic circuit, as described above, since the yoke is made of materials having the same thickness, magnetic saturation occurs at the bonded portion of the cylindrical peripheral portion 1a and the bottom yoke 1b and at the bent portion shown in FIG. 4. In order to prevent this problem, the materials for the yoke should have a sufficient thickness in accordance with the characteristics of the magnet. As a result, the yoke is increased in weight. In addition, as the recess 1c is formed in the bottom yoke 1b, the sectional area of the yoke decreases at this portion, and therefore, it is necessary to use a thicker plate to compensate for the decrease.
The present invention aims to provide a light-weight, high-efficiency magnetic circuit for loudspeakers, and a loudspeaker using the circuit.